When not on Windows, the `configureFlags` attr of `libmpc` is set to `(drv.configureFlags or [])`. Since `drv.configureFlags` seems to be equal to `["--enable-static" "--disable-shared"]` (at least on my machine), `configureFlags` appears to be merged with itself and in the end, it is equal to: `["--enable-static" "--disable-shared" "--enable-static" "--disable-shared"]`. You can verify this by comparing the derivations of `libmpc` with and without this patch. This changes the derivation of `libmpc` as well the derivation of all dependent packages, like `gcc`, causing cache misses from https://cache.nixos.org/. Because these packages are built and stored in the IOHK cache, users don't notice this, until they use a different version of `nixpkgs` than the one pinned by `haskell.nix`, e.g., one with a different version of `glibc` so that the IOHK cache cannot be used. This results in `gcc` and other packages being built from scratch instead of being downloaded from https://cache.nixos.org/. Fix this by hoisting the conditional one level higher so that `overrideAttrs` isn't even called on non-Windows OSes. Do the same for `mpfr` for consistency. I haven't tested this on Windows. I'm not sure whether I have hoisted the conditional too high, see the first comment in the file.